The removal of foreign bodies from patients often requires the use of endoscopic devices. In particular, gastroenterologists commonly use grasping or crushing devices to extract stones from a patient's biliary duct. Additionally, snares are often used when removing stents or other foreign objects.
Grasping and crushing devices generally take the form of wire baskets that deploy to capture the stone to be extracted. These wire baskets may be used for lithotripsy if the stone is too large to be removed intact. Lithotripsy involves crushing the stone into fragments to facilitate removal from the duct. Effective performance of such devices requires the baskets to have enough flexibility to be inserted into the common bile duct. However, the baskets also must have a certain degree of rigidity to dilate the duct to facilitate stone capture. Often, the baskets are deployed using a retaining cannula. In this case, the cannula retains the basket in a retracted configuration during insertion into the bile duct. Once within the grasping region of a stone, the basket extends from the cannula and opens to capture the stone. In such a case, the basket must have enough stiffness to open the duct when removed from the cannula, without being so stiff that it is permanently deformed due to retention within the cannula.
Aside from deformation associated with dilating the duct or retention within the cannula, a common failure of conventional baskets occurs during lithotripsy when the baskets are subject to forces often in excess of 50 pounds. Under such force, the basket can become severely deformed, rendering it unsuitable for repeated use. Such repeated use is desirable because of the frequent occurrence of the need to remove more than one stone or other object at a time from the patient. Therefore, design of these devices includes focus on the durability of the basket in repeated use settings.
To repeatedly crush and retrieve foreign objects, a basket must be flexible enough to traverse tortuous anatomy, yet stiff enough to open within a duct, and strong enough to crush stones. A single wire construction may meet any one of these criteria, but typically cannot meet all three requirements for repeated dilation and lithotripsy. It has been proposed, therefore, to construct a retrieval basket of a stranded cable, such as stainless steel cable. Purely stainless steel cable (the core and strands) may work well for the extraction of a single stone, but is subject to the deformation problems discussed previously when used for repeated dilatation or lithotripsy.
Other baskets are formed from cable which includes a superelastic, sometimes referred to as shape memory, core wrapped with strands of stainless steel to surround the core. Nitinol is often used as the superelastic core in these devices. Nitinol is a specially heat-treated Titanium-Nickel (Ti--Ni) alloy, preferably approximately 55%/45% Nickel to Titanium (Ni--Ti). These baskets require heat treatment for the core to retain its shape. Such a configuration allows for some improvement in performance when the baskets are used repeatedly and for lithotripsy because the superelastic core better retains its shape.
However, superelastic materials of this type experience phase transformations when subject to a certain level of stress loading. Lithotripsy often reaches these stress levels. Upon a phase transformation, the core of the cable stretches, rendering the device incapable of transferring force to the stone to complete the crushing process. Furthermore, the superelastic alloy has a greater reversible elongation than do the surrounding stainless steel strands. This results in a difference in deformation between the core and the surrounding strands leading to a permanent deformation of the cable. Such deformation results in an alteration of the basket shape, making it less desirable to use for its intended purpose.
Moreover, manufacturing both the cable core and strands from superelastic alloy wires results in a cable that unwinds due to the highly elastic nature of the material. Thus, a retrieval basket of such cable also will not retain its desired shape without heat treating.